US5151800A

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Publication number: US5151800A
Application number: US07/627,984
Authority: US
Inventors: Juris Upatnieks
Original assignee: Environmental Research Institute of Michigan
Current assignee: ERIM International Inc
Priority date: 1990-12-17
Filing date: 1990-12-17
Publication date: 1992-09-29
Anticipated expiration: 2010-12-17

Abstract

A compact, self-contained monolith hologram display apparatus includes a laser diode mounted on an edge of a solid glass plate having a front surface with a hologram mounted thereon. The beam is expanded within the glass plate and is reflected off an opposing mirrored contoured edge surface that collimates the beam and directs it onto an angled reflection grating. The reflection grating is mounted onto an edge which is canted with respect to the front surface. The reflected beam diffracts from the grating and impinges upon the hologram to provide for reconstructed image that has achromatic properties.

Description

TECHNICAL FIELD

The present invention relates to an apparatus for achromatically displaying a hologram and more particularly to an apparatus for achromatically displaying holograms using edge illumination.

BACKGROUND OF THE INVENTION

Holograms for displaying visual information are being applied to an ever-widening range of fields including head-up displays, aiming sights, light collimators and light focusing apparatuses.

Previous methods of illuminating a hologram include illuminating the hologram from the rear surface thereof and transmitting the light through the hologram. This type of hologram is commonly referred to as a transmission hologram. The first transmission holograms transmit the reference beam on axis with the object beam. Later developments provide off axis transmission such that an observer views the image without directly looking into the reconstruction beam.

Reflection holograms have been developed which require a front light source. Even though reflection holograms eliminate the need for a light source behind the hologram thereby allowing a hologram to be mounted on a solid wall, much care has to be taken in choosing the appropriate light source angle so that the observer does not obstruct the light source and cast a shadow on the hologram. Furthermore the space in front of the hologram needs to be free of other obstacles that can cast interfering shadows.

Holograms, particularly glass plate holograms, can also be illuminated with an expanded laser beam that enters the side edge of the hologram cover plate. Side edge illuminated holograms are initially recorded with a reference beam entering the side edge of the hologram cover plate. One such side edge illumination system is disclosed in my U.S. Pat. No. 4,643,515 issued Feb. 17, 1987.

Expansion of the laser beams in ambient air has several disadvantages. Firstly, the space required to expand the beam renders the hologram display larger. The beam exiting a laser and travelling through air requires at least one lens, and several air-glass interfaces before the light enters the hologram edge. Each glass surface must be kept clean and free of moisture condensation in order to reconstruct the hologram. Keeping the hologram and each glass surface dry and dust free can be a serious problem in a holographic weapon sight application that is used in the field. Secondly, the laser used for expanding the laser beam in air is an electrically pumped gas or light pump solid laser that is large and often consumes more than 10 watts of input power at voltages of over 1,000 volts. The typical laser occupies space greater than 100,000 cubic millimeters.

Compact lasers such as laser diodes are known. The power consumption of a laser diode is often under 1 watt and operates with less than 5 volts. The volume of a laser diode is under a few cubic centimeters. Present laser diodes however emit a beam that has wavelength drift. The wavelength drift is undesirable for a clear accurate reconstruction of a hologram.

What is needed is a compact hologram display that is lightweight, has low power consumption, reduces exposed air-glass interfaces and sufficiently compensates for the wavelength drift of a compact laser diode. A system with the above advantages is also desired in an edge illuminated hologram display where the reference beam cannot be obstructed and the potential for shadows cast on the hologram is eliminated. Furthermore, a hologram display is desired that does not require extra space for the divergence of the laser beam.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a hologram display includes a solid glass or plexiglass transparent unit having a planar front surface with a hologram thereon, a planar rear surface, and edge surfaces therebetween. A laser diode is mounted directly on a first edge surface between the front and rear surfaces. The laser diode has a laser light beam diverging in a non-circular fashion within the unit. A greater divergence angle lies in a plane parallel to the front surface and a narrower divergence angle lies in a plane perpendicular to the front surface. The laser diode directs its diverging beam to at least one other edge surface of the plate before the beam impinges on and illuminates the hologram at the front surface.

In one embodiment the diverging laser beam reflects off a second edge surface which opposes the first edge surface. The second edge surface can have a curved contour and a reflective coating thereon which reflects and collimates the laser beam and directs the beam to the front surface at the hologram. Preferably, the con d second edge surface reflects and directs the beam to an additional edge surface which is adjacent and to the rear of the first edge surface. The additional edge surface has a reflection grating thereon and is angled with respect to both the first edge surface and front surface. The beam reflecting from the reflection grating impinges upon the hologram front surface. The frequency of the reflection grating is typically higher than the carrier frequency of the hologram depending upon the angle between the additional edge surface and hologram plane such that the wavelength drift of the laser diode from its predetermined wavelength is compensated by the achromatism caused by the combination of the reflection grating and hologram.

In accordance with the broader aspect of the invention, the hologram display apparatus includes a holographic surface and a substantially planar grating. The holographic surface and grating are canted with respect to each other. The hologram has a carrier frequency that is lower than the grating frequency such that chromatic dispersion about a light wavelength emanating from the laser diode is minimized through the hologram.

In accordance with another broader aspect of the invention, an illuminating light source for producing a reconstructive light beam for illuminating a hologram is optically mounted to the first edge surface of a transparent unit having a substantially uniform index of refraction therein. The light source can be mounted via an optical fiber or directly mounted on a surface of the unit. Preferably the light source produces monochromatic light. A holographic surface on or in the transparent unit can be planar or contoured into an arc or other geometric design for viewing over greater angles. The hologram can be laminated such that a holographic photographic plate is mounted against one surface of the cover plate. The plates both have substantially identical indexes of refraction.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference now is made to the accompanying drawings in which:

FIG. 1 is a top plan view of a hologram display according to one embodiment of the invention;

FIG. 2 is a front elevational view of the display shown in FIG. 1;

FIG. 3 schematically illustrates a prior art arrangement of a hologram and grating to form an achromatic system;

FIG. 4 illustrates an alternate embodiment according to the invention incorporating an angled transmission grating;

FIG. 5 is a schematic view illustrating the angles of incidence and refraction for the embodiment shown in FIG. 1;

FIG. 6 is a schematic view illustrating the recording system for the hologram in FIG. 1;

FIG. 7 illustrates an alternate hologram display using edge illumination and internal beam expansion;

FIG. 8 illustrates an alternate hologram display using multiple internal reflections;

FIG. 9 illustrates an alternate display using an angles reflective edge surface to reflect the beam directly to the hologram;

FIG. 10 illustrates an alternate display of a laser diode coupled to the hologram via optical fiber; and

FIG. 11 discloses an alternate hologram display having an opposing edge surface collimating the beam and reflecting the beam toward the hologram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to FIG. 1 and FIG. 2 which illustrate a monolithichologram display system 10. Thesystem 10 includes a developed photographicplanar plate 12 having a developed emulsion (i.e. hologram) 14 at itsback side 15. Thephotographic plate 12 is mounted onto a planarfront surface 18 of a solidglass cover plate 16. The

plates

12 and 16 abut each other and have similar indexes of refraction. An optical cement is between the two

plates

12 and 16 such that no air gap exists between the two plates. The optical cement has an index of refraction equal to or higher than the index of refraction forplate 16. The solidglass cover plate 16 has a planarrear surface 20 parallel to thefront surface 18 and a plurality of side edge surfaces 22, 24, 26, 28 and 30 between thefront surface 18 andrear surface 20.

Alaser diode 32 is mounted directly onedge surface 22 such that the reconstructivelight beam 34 from the laser diode is directed intoglass plate 16 throughedge surface 22.Laser diode 32 can be powered by batteries to achieve complete portability of thedisplay system 10. The laser diode desirably has a volume of less than a few cubic millimeters. Its power consumption is usually less than 1 watt and has an operating voltage under 5 volts. Thebeam 34 that emanates from the laser diode diverges in a non-circular, elongated pattern such that the divergence shown in FIG. 2 parallel to thefront surface 18 is substantially greater than the divergence transverse to thefront surface 18 shown in FIG. 1. The ratios between the two rates of diverging can be as large as five to one.

The divergingbeam 34 is reflected off of contourededge surface 24.Edge surface 24 has areflective coating 36 adhered thereto. Theedge surface 24 is contoured in both the vertical direction as shown in FIG. 2 and the horizontal direction as shown in FIG. 1 to collimate the divergingbeam 34 to form collimatedbeam leg 38.

Collimated beam leg 38 is directed parallel tofront surface 20 towardedge surface 26 which is adjacent to the rear ofedge surface 22. Furthermore,edge surface 26 is canted with respect to both theedge surface 22 andfront surface 18. Theedge surface 26 has a reflection grating 40 thereon which both reflects and diffracts thebeam leg 38. Thelast leg 39 ofbeam 34 is directed to passsurface 18, through the optical cement and to thephotographic plate 12 through thehologram 14 to reconstruct a virtual or real image. As shown in FIG. 1, the hologram is a transmission hologram that is observed from the plate glass 12 (viewed from below as shown in FIG. 1). However, the hologram can alternatively be a reflection hologram and can be viewed from the side with the glass plate 16 (viewed from above as shown in FIG. 1).

A conventional achromaticgrating system 41 is illustrated in FIG. 3. A transmission grating 43 is parallel to ahologram 45. The frequency of the grating and the carrier frequency of the hologram are equal to produce achromaticity between two wavelengths λ1 and λ2 which are slightly longer and shorter respectively about a predetermined measured wavelength λ. The two different wavelengths λ1 and λ2 are both diffracted a different degree through the grating and then are diffracted through the hologram such that the exit angles of λ1 and λ2 are equal and therefore the chromatic dispersion is greatly reduced or eliminated between the two different wavelengths.

Several disadvantages with the conventional system as illustrated in FIG. 3 exist. Firstly, the grating must be spatially displaced from the hologram in both a longitudinal and lateral direction. The grating must be at least as large as the hologram. The grating, by being as large as the hologram and spaced behind the hologram, must be laterally displaced a significant amount from the hologram. Consequently, a relatively large display apparatus is needed.

Analternate arrangement 44 shown in FIG. 4 illustrates the angles of incidence and diffraction of the light beam with the transmission grating 43a and the hologram transmission 14a. In this embodiment, the grating plane is canted with respect to the plane of the hologram 4a. The relationship is as follows:

λ f.sub.1 =sin θ.sub.2 +sin θ.sub.1     (1)

where f, is the carrier frequency of the hologram, λ is the wavelength of the light beam, θ₁ is the angle incidence, and θ₂ is the angle of diffraction (i.e. exit angle). We can find the rate of change of angle θ₁ with respect to change in λ by differentiating and solving for dθ₁ /dλ as follows: ##EQU1## where λ is the center wavelength of the light beam. Letting θ₂ approximately equal 0 then: ##EQU2##

For the grating in FIG. 4 we obtain the equation:

λf.sub.2 =sin θ.sub.3 +sin θ.sub.4      (4)

where f₂ is the frequency of the transmission grating 43a, θ₃ is the angle of incidence on grating 43a, and θ₄ is the angle of diffraction from the grating 43a. We assume the incident light at angle θ₃ constant. Differentiating and solving for dθ₄ /dλ we obtain: ##EQU3## One can now set θ₃ =θ₄ where the angle of incidence equals the angle of diffraction. One then obtains: ##EQU4## To be achromatic, the light illuminating hologram 14a and emanating from grating 43a must have its dispersion equal, in other words, the angular changes as a function of λ must be equal. Thus, the following condition is needed: ##EQU5## which as shown above is: ##EQU6## Upon finding the appropriate angles θ₄ and θ₁ based on λ, f₁, and f₂, one can see by simple geometry that the transmission grating 43a can be shorter than the hologram 14a.

A further reduction of space for displaying the hologram can be achieved if the transmission grating 43a is replaced by the aforementioned reflection grating 40. However, for a reflection grating 40, if θ₃ =θ₄ then angles θ₃ and θ₄ would coincide which is an impractical arrangement. If θ₃ is increased with respect to θ₄, we are able to obtain an incident ray which is approximately parallel to thehologram 14 as shown in FIG. 5. θ₄ is chosen to match the required incident angle θ₁ of the hologram at λ₀. The tilt of angle of the grating 40 is selected to satisfy Equation 7 for the geometry where theincident beam leg 38 is substantially parallel tohologram 14. The following four equations must be satisfied.

θ.sub.3 -θ.sub.4 =90°-θ.sub.1     (10)

λ.sub.0 f.sub.1 =sin θ.sub.1                  (11)

λ.sub.0 f.sub.2 =sin θ.sub.3 +sin θ.sub.4(4)

and ##EQU7## The derivatives in Equation (7) are substituted by terms in Equation 3 and Equation 5, to obtain: ##EQU8## Equation 12 has θ₃ substituted by terms found inEquation 10 to obtain: ##EQU9## A desired θ₁ is selected and the required angle θ₄ is thus calculated.

The achromatization of thehologram 14 when used for a range finder and aim sight can reduce aiming error by a factor of up to 45 times compared to a hologram with When tanθ₁ =10, the aiming error can be reduced by a factor of 10 times. These numbers are based on λ₀ =633 nm.

Referring now to FIG. 6, the hologram of FIG. 1 is produced by alaser light beam 60 aimed toward abeam splitter 62 which splitsbeam 60 into two

beams

64 and 74.Beam 64 passes through alens 66 to produce a divergingbeam leg 68 which impinges on anobject 70. Anobject wavefront 72 reflects offobject 70 situated to the left of thehologram assembly 10. Thewavefront 72 is then directed through theglass plate 16 to impinge upon anemulsion layer 13. Thesecond beam 74 is reflected offmirror 76 and through a focusinglens 78 which focuses the beam atpoint 80 at theedge surface 22. Thelens 78 can be astigmatic to achieve a proper beam spread within the glass plate in the vertical and horizontal directions. Thebeam 74 is reflected off mirroredcoating 36 on contourededge 24 to produce the collimatedbeam leg 82 which in turn is reflected off of reflection grating 26 such that areference wavefront 84 is directed onto theemulsion 13. An optical coupling liquid is interposed between the

plates

12 and 16 to fill up the space therebetween. The liquid has an index of refraction equal to or slightly higher than the

glass plates

12 and 16. If a reflection hologram is being produced, theobject 70 will be placed to the right ofplate 12 and the

beams

64 and 68 are similarly directed to the right to impinge onobject 70 to form anobject wavefront 72 impinging onplate 12 from the right.

After the proper exposure to

wavefronts

72 and 84, thehologram plate 12 is taken offplate 16 and theemulsion 13 is then processed to formhologram 14. The processedplate 12 is remounted onplate 16 with optical cement for display of the hologram. The path of thereference beam 74 within theglass plate 16 shown in FIG. 6 is duplicated by the path ofbeam 34 when it reconstructs the image of theobject 70 shown in FIG. 1.

Other geometries are possible for a monolithic edge illuminated hologram display system. As shown in FIG. 7,laser diode 32 is mounted onside edge surface 22b near therear surface 20 ofglass plate 16b. The opposingside edge surface 24b has areflective coating 36b. Theedge surface 24b is flat so that thebeam 34b continues to diverge along itsleg 38b after reflection off ofedge surface 24b until it impingeshologram 14. For certain applications, this simplified geometric arrangement is adequate with the divergingbeam 38 impinging uponhologram 14.

Referring now to FIG. 8, an alternate arrangement is shown in which thelaser diode 32 is mounted near a front portion ofedge surface 22c adjacent thehologram 14. Thediode 32 has a beam 32c arranged such that it is internally reflected off theback surface 20 of theplate 16c. The angle of incidence ofbeam surface 20 is sufficiently great to satisfy Snell's equation for complete internal reflection. Therefore, total internal reflection occurs without the need for a reflective coating onback surface 20.Edge surface 24c has a reflective coating thereon.Edge surface 24c is flat, such that theleg 39c ofbeam 34 continues to diverge after reflection offedge surface 24c until it impinges upon thehologram 14.

Another embodiment illustrated in FIG. 9 discloses alaser diode 32 mounted at the center of theedge 22d. The opposingedge surface 24d is angled such that it directs divergingbeam leg 38d toward thehologram 14 at thefront surface 18d.

FIG. 10 discloses an arrangement and geometry which provides for longer beam travel within the glass plate. Thelaser diode 32e is remotely positioned from theedge surface 22e and is optically coupled thereto by anoptical fiber 50 mounted atentrance point 52. Theentrance point 52 is located near the front surface 18e. Thebeam 34e diverges from theentrance point 52 and is aimed directly atedge surface 24e at an angle toward therear surface 20e. The beam is reflected byreflective coating 36e onedge surface 24e onto therear surface 20e. Total internal reflection off ofrear surface 20e then redirects the beam back towardedge surface 22e.Edge surface 22e has a reflective coating thereon except for atransparent window 54 at theentrance point 52. The incident beam is then reflected off ofedge surface 22e and directed towardhologram 14. In this geometry, the beam is reflected off of three surfaces within the glass plate before it impinges onhologram 14. Thebeam 34e also travels farther, thus its diverging angle is smaller than the one shown in FIG. 9.

FIG. 11 discloses an arrangement which has thelaser diode 32f mounted onside edge surface 22f near thefront surface 18f. Thebeam 34f is directed to edge surface 24f. Edge 24f is both angled and contoured such that thebeam leg 38f is both collimated and directed toward thehologram 14 as it reflects off of edge surface 24f.

Depending upon the desired geometric configuration of the alternate holograms, as depicted in FIGS. 7-11, the arrangement of the hologram recording system as illustrated in FIG. 6 is modified such thatreference beam 74 is reflected offmirror 76 and throughlens 78 to focus on theside edge surface 22 and pass into and diverge in the hologram assembly to duplicate the same path as illustrated in FIGS. 7-11.

In this fashion, depending upon the need for chromatic correction and precision of a collimated beam onto thehologram 14, various geometries are available for a compact monolith hologram display in which the beam emanates from a side edge surface of the glass plate and is expanded within the glass plate before it impinges upon the hologram on the front surface of the glass plate. Furthermore, if chromatic correction is required, a diffraction grating can be mounted on a canted edge of theglass plate 16 to reflect a beam that has been expanded within the glass plate. Furthermore, if a collimated beam is required, one of the edges can be contoured and surfaced with a reflective material to both reflect and collimate the expanded beam. The contoured shaped reflective surface can be substituted with a holographic diffraction reflection grating to form a collimated beam.

Furthermore, other geometries can include thefront surface 20 of the hologram being contoured to provide wider viewing angles of the hologram.

Theemulsion 13 and derived processedhologram 14 can also be applied or coated directly onplate 16 thereby eliminatingphotographic plate 12.

Other variations and modifications of the present invention are possible without departing from its scope and spirit as defined by the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege as claimed are defined as follows:

1. A hologram display apparatus characterized by:

a transparent plate with a pair of opposing surfaces and edge surfaces therebetween;

one of said opposing surfaces having a hologram thereagainst;

an illuminating light source for producing a light beam for illuminating said hologram: and

said illuminating light source being optically mounted on one of said edge surfaces of said transparent plate such that expansion of said light beam from said source occurs within said transparent plate, and said illuminating light source further directing said light beam to reflect against at least one surface of said transparent plate before illuminating said hologram at said one of said opposing surface.

2. A hologram display apparatus as defined in claim 1, further characterized by:

said at least one surface being a reflective edge surface;

said reflective edge surface having optical collimating means for collimating a diverging light beam from said illuminating light source.

3. A hologram display apparatus as defined in claim 2 further characterized by:

said reflective edge surface opposing said first edge surface having a reflective coating thereon;

said opposing reflective edge surface being contoured and angled to collimate and direct a light beam to said one of said opposing surfaces.

4. A hologram display apparatus as defined in claim 2 further characterized by:

a reflection grating on another of said edge surfaces to receive said collimated beam from said reflective edge surface to reflect said collimated beam onto said hologram on said one of said opposing surfaces for achromaticity of said hologram approximately at a predetermined wavelength of light.

5. A hologram display apparatus as defined in claim 4 further characterized by:

said reflection grating being canted with respect to said one of said opposing surfaces.

6. A hologram display apparatus as defined in claim 1, further characterized by:

said illuminating light source being a diode laser.

7. A hologram display apparatus as defined in claim 6 further characterized by:

said at least one surface being reflective and contoured to collimate a diverging light beam from said diode laser.

8. A hologram as defined in claim 7 further characterized by:

said at least one surface being one of said edge surfaces.

9. A hologram display apparatus as defined in claim 6 further characterized by:

said at least one surface having a holographic diffraction reflection grating to collimate a diverging light beam from said diode laser.

10. A hologram as defined in claim 9 further characterized by:

said at least one surface being one of said edge surfaces.

11. A hologram display apparatus as defined in claim 6 further characterized by:

said diode laser being mounted directly on said one of the edge surfaces.

12. A hologram display apparatus as defined in claim 1 further characterized by:

said light beam directed to internally reflect off said rear surface and to reflect off a reflective opposing edge surface that opposes said one of said edge surfaces.

13. A hologram display apparatus as defined in claim 12 further characterized by:

said light beam being reflected from said reflective opposing edge surface and directed to said first edge surface to which said light source is optically mounted, said one of said edge surfaces having a reflective coating thereon with a window to let in said light beam from said light source.

14. A hologram display apparatus characterized by:

a substantially planar transparent plate having first and second major surfaces that are parallel to each other, and side edge surfaces between said first and second major surfaces, said side edge surfaces including a canted side edge surface neither parallel nor perpendicular to either said first or second major surface;

a substantially planar hologram disposed on said first major surface of said transparent plate;

a substantially planar diffraction grating disposed on said canted side edge surface of said transparent plate;

said hologram having a carrier frequency and said diffraction grating having a different frequency selected with regard to a predetermined wavelength of light for causing light diffracted by said diffraction grating and by said hologram to be achromatized about said predetermined wavelength of light.

15. A hologram display apparatus as defined in claim 14 further characterized by:

said diffraction grating being a reflective diffraction grating.

16. A hologram display device as defined in claim 15 further characterized by:

a light collimator being mounted on a side edge of said transparent plate opposing said canted side edge surface with said reflective diffraction grating.

17. A hologram display apparatus characterized by:

a solid transparent unit having first and second distinct and spaced surfaces;

a hologram mounted on said first surface of said solid transparent unit;

a reflective diffraction grating being mounted on said second surface of said solid transparent unit, said second surface of said solid transparent unit disposed at an angle with respect to said first surface of said solid transparent unit whereby said reflective diffraction grating receives a reconstructing light beam that passes by said hologram and substantially parallel thereto to reflect and diffract said light beam so that it impinges onto said hologram; and

said hologram diffracts said light beam from said reflective diffraction grating such that the final direction of said light beam near a predetermined center wavelength is independent of the wavelength of said beam.

18. A hologram display apparatus characterized by:

a solid transparent member with first and second opposing surfaces and edge surfaces therebetween;

a hologram on the first of said opposing surface of said plate;

a substantially monochromatic light source for producing a reconstructive light beam for illuminating said hologram;

said monochromatic light source optically mounted on one of said edge surfaces of said member such that expansion of said light beam from said light source occurs within said member with said dispersion being substantially greater along an axis parallel with said first opposing surface and less along an axis perpendicular to said first opposing surface;

a second of said edge surfaces of said plate opposing said one of said edge surfaces and having a reflective light collimator thereon to reflect and collimate said dispersing light beam from said one of said edge surfaces and directing the collimated beam of light behind and substantially parallel to said first opposing surface;

a third of said edge surfaces adjacent to and canted at an angle with respect to said first opposing surface and said one of said edge surfaces;

said third of said edge surfaces having a reflective grating to reflect said collimated beam and direct it to impinge on said hologram; and

said reflective grating having a frequency different from the carrier frequency of the hologram and angled with respect to the hologram such that achromaticity about a predetermined wavelength of light from said light source is provided.

19. A hologram display apparatus as defined in claim 18 further characterized by:

said substantially monochromatic light source being a laser diode.

20. A hologram display apparatus as defined in claim 19 further characterized by:

said laser diode being mounted directly on said first edge surface.

21. A method of recording a hologram comprising;

placing a recording medium sensitive to electromagnetic energy adjacent a front surface of a transparent body having said front surface, a rear surface, and edge surfaces therebetween;

forming a first and second beam from a coherent source of electromagnetic energy;

expanding said first beam;

creating an object wavefront by illuminating an object to be holographically recorded with said expanded first beam;

directing a portion of said object wavefront to impinge on said recording medium;

creating a reference wavefront by directing and focusing said second beam approximately at one of said edge surfaces;

diverging said second beam within said transparent body;

reflecting said expanded second beam off of one of said surfaces of said transparent body to form said reference wavefront; and

directing said reference wavefront to impinge on said recording medium and creating an electromagnetic interference pattern with said object wavefront;

rendering said interference pattern permanent in said recording medium.

22. A hologram display apparatus comprising:

a transparent member having a substantially uniform index of refraction therein;

a hologram disposed on a first surface of said transparent member;

a reflective diffraction grating disposed on a second surface of said transparent member;

a light source for illuminating said hologram with a reconstructive light beam that expands within said transparent member, reflects and diffracts from said reflective diffraction grating to impinge on said hologram to form a holographic image.

23. A hologram display as defined in claim 22 further characterized by:

said transparent member having a reflective surface for reflecting said reconstruction beam therein.

24. A hologram display apparatus as defined in claim 22 further characterized by:

said light source being mounted to said transparent member.